How many significant figures does $0.6413790$ have?
Explanation: Identify the non-zero digits and any zeros between them. These are all significant. $0.{641379}0$ Leading zeros are not significant. $\color{orange}{0.}{641379}0$ Since there is a decimal, trailing zeros are significant. $\color{orange}{0.}{6413790}$ Counting all the significant digits gives us 7.